1. Field of the Invention
The invention relates to a transparent, dyed cooktop comprising a glass ceramic having high quartz mixed crystals as the predominant crystal phase as well as a method for the production thereof.
2. Description of Related Art
Cooktops having a glass-ceramic plate as a cooking surface are familiar prior art. Such glass-ceramic plates are usually present as flat plates or are shaped three-dimensionally.
Glass ceramics having high quartz mixed crystals as the predominant crystal phase are produced from crystallizable lithium aluminum silicate glasses.
These glass ceramics are produced in several steps.
In the large-scale production of glass ceramics, first the crystallizable initial glass of a mixture of shards and powder-form raw materials is melted at temperatures usually between 1500 and 1650° C. Typically, arsenic oxide and/or antimony oxide is used as the refining agent in the melt. These refining agents are compatible with the required glass-ceramic properties and lead to good bubble qualities of the melt. Although these materials are firmly bound in the glass structure, they are a disadvantage, however, from the aspects of safety and environmental protection. Special precautions must be implemented in producing and processing raw materials due to evaporation in the melts.
Recently, the use of SnO2 is described in particular as an unobjectionable refining agent. In order to obtain good bubble qualities, in addition to SnO2, halide compounds are preferably used as additional refining agents at conventional melting temperatures (maximum of approximately 1680° C.). Thus, in the Japanese Applications JP 11 100 229 A and JP 11 100 230 A, the use of 0.1-2 wt. % SnO2 and 0-1 wt. % Cl has been described. According to these publications, coloring is obtained by the addition of V2O5 as a single coloring agent.
The addition of 0.05-1 wt. % fluorine (US 2007 0004578 A1) and 0.01-1 wt.-% bromine (US 2008 0026927 A1) for supporting the refining with SnO2 is also disclosed. Refining temperatures below 1700° C. are also described in these publications. The main coloring agent is V2O5. The addition of halides is a disadvantage, since they evaporate intensely at the melting temperature and thus form toxic compounds, such as HF, for example.
The use of SnO2 in combination with high-temperature refining above 1700° C. in order to obtain good bubble qualities is described in DE 199 39 787 C2. This step, however, provides no hint with respect to obtaining good display capability in the wavelength range starting from 450 nm.
After melting and refining, the glass usually undergoes a hot forming by rolling or recently by floating, in order to produce plates. For an economical production, on the one hand, a low melting temperature and a low processing temperature PR are desired, while on the other hand, the glass should not show any devitrification during the shaping. This means that disruptive crystals that adversely affect strength in the initial glasses and in the glass ceramics produced therefrom must not be formed. Since the shaping takes place in the vicinity of the processing temperature PR (viscosity 104 dPas) of the glass, it must be assured that the upper devitrification temperature of the melt lies in the vicinity of and more favorably below the processing temperature, in order to avoid the formation of disruptive crystals.
Subsequently, the initial glass is converted to the glass-ceramic article by controlled crystallization. This ceramicizing is produced in a two-step temperature process, in which seeds are produced first, usually from ZrO2/TiO2 mixed crystals, by nucleation at a temperature between 680 and 800° C. SnO2 may also participate in the nucleation. With a subsequent temperature increase, the high quartz mixed crystals grow on these seeds. High rates of crystal growth, which are desired for an economical, rapid ceramicizing, are obtained at temperatures of 850 to 950° C. At this maximum production temperature, the structure of the glass ceramic is homogenized and the optical, physical and chemical properties of the glass ceramic are adjusted. If desired, the high quartz mixed crystals can still be converted subsequently into keatite mixed crystals. The transformation into keatite mixed crystals is produced with a temperature increase to a range of approximately. 950 to 1200° C. With the conversion of high quartz to keatite mixed crystals, the thermal expansion coefficient of the glass ceramic increases and the transparency is reduced due to the light scatter accompanying the enlargement of the crystals. As a rule, glass ceramics having keatite mixed crystals as the main phase are thus translucent or opaque and the light scatter associated therewith acts negatively on display capability.
A key property of glass ceramics having high quartz mixed crystals as the main crystal phase is the ability to produce materials that provide an extremely low heat expansion coefficient of <0.5×10−6/K in the range of room temperature to 700° C. and above. Based on the low thermal expansion, these glass ceramics possess an excellent strength at different temperatures, and an excellent stability with temperature changes.
With application as cooktops, based on the requirements from practical use, technical development leads to very specific, partially contradictory requirements for transmission.